1. Field of the Invention
The present invention is generally related to improvements in electric motor drive circuits and, more particularly, to an anti-stall drive circuit for a permanent magnet d.c. motor which is operated at very slow speed. The invention has particular application in railway data recorders of the type that use a closed-loop magnetic tape data pack.
2. Description of the Prior Art
Railway recorders are installed on locomotives to record various parameters of operation including speed, elapsed time, brake application and the like. These recorders are analogous to flight recorders on commercial aircraft and allow for analysis of the events which occured prior to an accident. In addition, the recorders may be used to verify rules compliance by monitoring daily operations over a specific district or on a system wide basis.
Early railway recorders used chart paper and included one or more pen styli to make the recording on the chart paper. These early recorders have now been generally replaced by magnetic tape recorders which use closed-loop magnetic tape data packs. The data packs, which are similar to the familiar audio eight-track tape cartridges, eliminate the need for frequent reloads and the risk of running out of paper. And because of their cartridge-type packaging, the data packs can be easily removed and replaced by unskilled railway personnel. The recorded data may be "readout" and plotted on chart paper specially designed for the purpose by means of a portable playback unit simply by removing the data pack from the recorder and inserting it into the playback unit.
Railway recorders and playback units of the type described are manufactured and sold by Pulse Electronics, Inc., of Rockville, Maryland. A specific example of such equipment is the 48H Series Locomotive Recording System. The recorder in this system uses a permanent magnet d.c. motor to drive the capstan and is characterized by a very slow capstan speed. The slow capstan speed, and hence tape speed, is justified by the fact that events are being recorded, and therefore a wide frequency bandwidth is not required, and it is desirable to maximize the elapsed time that can be recorded on the tape loop of the data pack. The slow speed of the capstan is accomplished in part by a reduction gear train, but this alone was found not to produce a slow enough tape speed. Therefore, the motor drive circuit was specially designed with tachometer feedback to produce the desired slow tape speed.
The problem with a very slow operation of a permanent magnet d.c. motor is that the motor appears to develop a high impedance contamination between the commutator and the brushes. At higher speeds, such contamination does not develop presumably because the rotor turns fast enough to either prevent the contamination from developing or to clean the contamination off the commutator as it forms. Unfortunately, no such self-cleaning action occurs on the commutator of a permanent magnet d.c. motor operated at very slow speeds in the locomotive recording system. As a result, there is occassionally experienced a stoppage of the motor due to a lack of electrical conduction between the commutator and the brushes. This is easily remedied by spinning the rotor of the motor, but in the meantime valuable data may have been lost.
What was needed was a way in which the stopped condition of the motor could be automatically detected and the motor restarted so that there would be no loss or only a minimal loss of data. The problem appears to be unique to the very slow speed operation of permanent magnet d.c. motors, and no solution was known in the prior art. Moreover, a.c. motor control circuits of the type which sense motor torque to develop a control signal such as shown in U.S. Pat. No. 3,519,910 to Pfaff et al. are not directly applicable to this particular problem. Other tape speed control circuits using a.c. motors such as shown in U.S. Pat. No. 3,634,744 to Toensing et al. and U.S. Pat. No. 3,800,200 to Bunting fail to suggest any remedy to the problem.